The book offered to readers is devoted to the presentation of the results of the theory of nuclear reactions at low energies, which have been obtained over the past twenty-five years. The continued success of the use of Breit-Wigner resonance formulas for the analysis of a large number of all kinds of nuclear reactions contributed to the emergence of a number of formulations of the theory of nuclear reactions, in which the simplest resonance formulas were substantiated, refined and generalized.